Applications of superconducting magnet systems include magnetic resonance methods. In order to achieve good resolution in such a method, the magnetic field in a sample volume must have a high degree of homogeneity. The basic homogeneity of the superconducting magnet can be optimized with the geometric arrangement of the field-generating magnet coils.
Frequently, gaps are provided in the magnet coils (so-called notch structures), in which no wire is wound, in order to improve the homogeneity of the magnetic field. As a result, however, valuable space for magnet windings is lost, which makes the magnet more expensive and increases the stray field. In an arrangement according to U.S. Pat. No. 6,617,853 B2, a superconducting magnet for high-resolution spectroscopy is made more compact in that one or more soft magnetic rings, which adopt the role of certain notch structures in the magnet coils, are provided.
The z-component of the magnetic field of an arrangement according to U.S. Pat. No. 6,617,853 B2 can be expanded in the sample volume in a series of spherical harmonics:
                    B        z            ⁡              (                  r          ,          z          ,          φ                )              =                  ∑                  n          =          0                ∞            ⁢                        ∑                      m            =            0                    n                ⁢                                            P              n              m                        ⁡                          (                              z                                                                            r                      2                                        +                                          z                      2                                                                                  )                                ⁢                                    (                                                r                  2                                +                                  z                  2                                            )                                      n              /              2                                ⁢                      (                                                            A                  nm                                ⁢                                  cos                  ⁡                                      (                                          m                      ⁢                                                                                          ⁢                      φ                                        )                                                              +                                                B                  nm                                ⁢                                  sin                  ⁡                                      (                                          m                      ⁢                                                                                          ⁢                      φ                                        )                                                                        )                                ,where, according to design, the coefficients Anm with m≠0 and all coefficients Bnm disappear. Due to manufacturing tolerances in the magnet arrangement, the coefficients Anm and Bnm differ from the calculated value. Shim coils, which can each be energized with their own current, are normally used to correct these non-disappearing coefficients. In the case where the coefficients differ greatly from their desired value, it may be that the current needed in certain shim coils is too high and the magnetic field of the magnet arrangement cannot be corrected as required. Alternatively, it may be that the problematic coefficient in the expansion of the magnetic field in a series of spherical harmonics cannot be corrected as no shim coil is provided for it. In such a situation, an expensive repair to the magnet system, in which part of the magnet arrangement has to be replaced, is necessary.
Suitable field shaping devices made from soft magnetic material are provided in order to improve the magnetic field homogeneity without winding new magnet coils.
Different types of field shaping devices made from sheet metal or foil are described in DE 10 16 505 A1. Special cases, in which the sheet metal or foils have cut-out rectangular windows, are specified in JP 4384220 B2. Non-continuous holes in the sheet metal are also disclosed in DE 10 2012 220 126 A1.
Specially formed field shaping devices can also be provided in order to dispense with the notch structures in superconducting magnet coils described above in order, in turn, to make the magnet more compact, as explained in DE 10 104 054 C1, for example.
A further aspect of field shaping devices is discussed in JP 3737636 B2: The saturation magnetization of the soft magnetic material is temperature-dependent. This dependency is particularly pronounced at high temperatures, such as room temperature for example. Due to the variable saturation magnetization, small temperature variations of the field shaping device then cause a change in the field in the working volume, which can adversely affect the NMR measurement.
To counteract this, it is proposed to accommodate the field shaping device in the He tank. The ideal prerequisites for stable conditions are created due to the low temperature of the field shaping device and its cooling by the liquid helium. However, as the field shaping device can only be sized after measuring the field in the working volume, the cryostat must be warmed up and completely dismantled after a first magnet test before the field shaping device can be mounted in the helium tank. Such an operation costs time and money.
As a possible solution, DE 10 2012 220 126 A1 also proposes cooling of the field shaping device to the temperature of the liquid helium in order to improve the magnetic properties of the field shaping device. As an alternative to this, it is proposed in DE 10 2012 220 126 A1 that the field shaping device be accommodated in a region of the magnet arrangement which is at room temperature, so that, in the operating state, the components of the field shaping device are easily accessible from outside the arrangement and can be modified without warming up the magnet coil system.
As an alternative solution, JP 3737636 B2 proposes that the field shaping device be glued to the inner tube of the nitrogen tank which encompasses the He tank. This inner tube is provided for keeping the radiation of the room temperature inner tube away from the He tank and is therefore also referred to below as a radiation shield inner tube. With a suitable design of the cryostat, it is only necessary to warm up and not completely dismantle the cryostat following a first magnet test in order to mount the field shaping device. Unfortunately, however, this solution has a major disadvantage, namely, when changes occur to the nitrogen filling level, the nitrogen tank moves, and with it, naturally, also the inner tube with the fitted field shaping device. This movement then results in a considerable variation in the magnetic field homogeneity in the working volume, which is unacceptable in many applications.